Even though here in Austin we don't have a lot of days when heat is needed, I think that I do need a functioning heater in the truck.  For one thing, I might need to be able to blow hot air on the windshield if it starts to fog up.

So, I removed the original hot water heater core and modified it by adding an electric ceramic heater element.  Keep reading for the details ...

To create an electric heater, I took a ceramic heater core from an electric space heater and inserted it into the existing hot-water based heater core from the truck.  The ceramic heater cores are readily available.  Since they are just resistors, they will work on AC (house) or DC (truck battery pack).  And I understand that the resistance increases as they get hotter so they are somewhat self protecting (I have not tested this myself).

I will use a relay to switch the traction battery pack voltage to the electric heater.  I think I have figured out how to wire it in with the existing ventilation system wiring so that it will only heat if the fan is turned on which helps keep me from burning stuff up.

First of all, I don't want to take credit for this method.  I have seen several web sites that used a variant of this method so I studied to see what other people have done, and then created my own solution.

To start, I obtained a cheap electric space heater from Amazon for about $20.  Here is the one I used, but I think that most of the 1500w ceramic space heaters use a very similar element, so almost any would do.

Ceramic heater core from off-the-shelf space heater
Ceramic heater core from off-the-shelf space heater

Here is what the heater core looks like after it has been removed from the housing.  When I removed it, I cut the wires as long as possible.  Also, there is a thermal circuit breaker in the housing next to the heater element, so I also saved this to use later.  You can see it attached to the brown wire on the right in the photo below.

Ceramic heater element removed from space heater
Ceramic heater element removed from space heater

Next, I laid the element on top of the original hot-water heater core that I earlier removed from the truck.  I traced the outline just by gouging the fins a little bit with a screwdriver, so I can see where to cut.

Marking where to cut the original heater core
Marking where to cut the original heater core

I then cut a rectangular piece out of the center of the original heater core.  I have read about various techniques to do this, but I just used a 4-1/2 inch angle grinder with a metal grinding wheel.  It made short work of the heater core and went right through it.  I had to use a hacksaw blade to finish out the corners and make them square.

Here is what it looks like after I cut the hole in the heater core.

Original heater core with center cut out
Original heater core with center cut out

And here, I laid the ceramic element in the heater core to test fit.

Test fitting the ceramic heating element
Test fitting the ceramic heating element

I wondered about how to handle the wires, and then I noticed that the original piping that was used for water came out the side, and it was the perfect size for routing wires.  So I drilled out one of the original water pipes (inlet or outlet, I don't know).

Drilled hole for wiring in location of water inlet-outlet
Drilled hole for wiring in location of water inlet-outlet

Here is the ceramic element, test fit again, this time showing how the wires will be routed.  By routing the wires this way, they will come out of the heater core the same way as the original hoses, which means I can just insert this back in the dash where it came from without needing any modifications to anything under the dash.

Test fitting the heater element with wire routing
Test fitting the heater element with wire routing

Okay, next question, how is the ceramic heater element attached to the original heater core.  I have seen several solutions.  One was to make a metal plate that covers the heater core and has places to screw the heater element.  Another was to use high temperature silicon.

I decided to try JB Weld.  It is a very strong epoxy that can handle high temperatures.  I have to say that before this experience I really had not used JB Weld before, but I have to say, that stuff is magical and I will be sure to keep some on hand from now on.

It took me three "epoxying" sessions to get this finished.  First I covered one side of the heater core with a piece of plastic and then laid it with the plastic side down.  This allowed me to place the ceramic element in the hole and then I squirted JB Weld all around the edge.  Once this cured, the ceramic element was securely glued to the heater core.

The next problem is that now there is a gap where the wires are connected to the heating element (take a look at the photo above).  I need some way to cover this.  I thought about a piece of plastic, or maybe using some metal sheeting.  I was worried about the temperature limitations of the plastic, and the electrical conductivity of a  metal sheet.  Then I thought about trying to make a piece out of JB Weld.  I found a piece of plastic packaging that almost the perfect size to use as a form, and I "cast" a piece using JB Weld.

Now that I can block off the area of the wiring, the last issue is how to force all the air through the ceramic element and to not bypass it by flowing through the remaining section of the heater core.  JB Weld to the rescue again.  This time I mixed some up and "painted" it on the fins, effectively sealing them up.

All of this took me about 3 sets of JB Weld, which was about $15.  The photo below shows the finished result.  You can see the ceramic element glued in the center.  The messy edge is because I did not have a tight fit when I poured the original JB Weld and it oozed out past the surface.  But this is only cosmetic and does not affect the function.  In the photo you can also see the piece that I cast, and the JB Weld covering the remaining original heater core.

Finished electrical heater core
Finished electrical heater core

Here is another photo from the other side.

Ceramic heater element permanently installed in heater core
Ceramic heater element permanently installed in heater core

Since I took the last photo, above, I have also added the thermal circuit breaker from the original space heater.  I attached it to the wiring with zip ties. This will be the inlet side, so as long as air is flowing it should remain cool.  But if air stops flowing and the heater element is still on, it should heat up the thermal breaker pretty quickly and cut power to the heater element.




Posted on Wed 13 February 2013

I have since discovered what happens when you try to break a circuit that has DC current at high amps. If the switch is not meant to break DC current it tends to arc and melt things. Therefore I doubt that the thermal breaker will work the way I intended. It may break once but will be ruined after that, or it may not break the circuit at all. Lets hope I don’t need it because I have no plans to take it apart and redo it now.



Posted on Sun 02 November 2014

I have used this through two winters now (and getting ready for a third) and it works great. In fact it can make the cab pretty warm in a short period of time. I wired it so that all the elements come on together, but it might be a good idea to be able to set it up as a two stage heater. However that would require a second relay circuit.


Dustin L

Posted on Thu 12 February 2015

Please tell more about how you wired it and pics will help



Posted on Sat 21 February 2015

I'm sorry I don't have anything readily available to show the heater wiring. Now that everything is installed I can't really take a photo. I will try to make a diagram to show what I did and post something on that.

I will describe it briefly. The heater core is powered directly from the traction pack. It gets about 140 VDC when it is turned on. I have a high voltage relay that switches power to the heater core. You have to be careful with the relay. Make sure it can actually handle the high DC voltage. Also, I learned the hard way what happens when you try to break a high current (~14A) DC circuit. The first relay I used was vaporized when I opened the circuit. So I selected a better relay (I'll try to find the part number) and also used something called a "quencharc" that helps keep things from frying when the relay is opened.

The relay is turned on or off from a switch in the cab. I repurposed the old A/C compressor switch. Since I don't have air conditioning any more, this switch is available. It also has the handy feature that it already went to a relay under the hood so I didn't need to change any wiring under the dash for that. It also happens to only be enabled when the blower fan is running so if I ever turn off the blower, the heater will be turned off too. To figure out how to do this I had to purchase wiring diagram for my truck which I found at some online site that sells such things. It was not very expensive. Once I had the original wiring diagrams I was able to see how to use the compressor switch this way.

I found that when the heater core is turned on, it initially draws about 15A at 140V and drops to about 12A as it warms up.



Posted on Sun 05 June 2016

After doing some research (including your page), I decided to use a 1500w inverter (12vDC-110vAC) to power my heater core. It worked very nicely on the bench, but I neglected to push air through it. The core maintained about 400 degrees Fahrenheit for ten minutes before I noticed that it was slowly melting its way down the housing. I was able to rescue the housing but needed a way to securely mount the core. I want to check to be sure that JB Weld is both non-conductive and can withstand 350 degrees (the average temperature with airflow).



Posted on Wed 08 June 2016

Thank you for your comment.

I had the same concerns as you. First I was concerned about electrical resistance. I just used a multimeter to see if I could measure any resistance at all. I jabbed probes into it and close together so there was not much JB Weld material between them. I never could make any measurement this way so my conclusion was the stuff is pretty much not conductive, certainly much higher resistance than the heater elements, so I decided not to worry about that.

As for heat, I did not do any tests on this. I agree that the heater element gets very hot without any air flowing. The way this is wired in my dash, the heater element cannot come on unless the blower fan is running. So the way I manage this is to just be careful and never run the heating element without fan. When I don’t want the heater any more, I turn off the heater element first and let the blower continue to run for a minute or two so it can cool off. I haven’t looked at it since I installed it to see if it has melted at all - I would have to disassemble my dash again. When I use the heater, I have not noticed any weird smells like plastic melting, if that counts for anything.

For this circuit, the hardest thing for me was to get the switching on/off to work correctly. My first attempt results in a nicely fried relay. I now have two relays, one on high side and one on low side. They are meant to switch high current DC and have magnetic blowouts. Also there is something called a quencharc wired in parallel. This is to make sure that the relay can open the high current DC (~12A at ~140 VDC) without arcing and destroying the relay. I have a switch on my dash to let me switch the relay (and hence the heater element) on and off. And there is an interlock with the fan as I mentioned before.

The other thing is, the heater element has a over temperature switch that is supposed to open if it gets too hot. This was part of the original space heater and I left it in there. I was thinking it could trip if the heater element ever gets too hot. However, I now realize it is designed for use with AC current and I doubt it would work at all with the DC. So I suspect if it tried to open it would either fuse or vaporize.

Good luck.



Posted on Fri 11 February 2022

Could you give details of how you wired up the heating element? I linked to diagram but instead of lights replace with the elements.



Posted on Wed 30 March 2022

Are you asking about how to hook up a heater instead of lights? or are you asking how to hook up your lights, similar to the way I did the heater?

Keep in mind it has been over 10 years since I worked on this. First I refer you to this post (the post of this comment):


The post itself does not have a lot about wiring but there is some discussion about it in the comments.

Assuming you are talking about hooking up a resistive heater core like I did, you need high voltage instead of the 12V. The heater core from household space heater is meant to run from 120 V AC. The core itself does not care if it is AC or DC since it is just a big resistor. So I used 140 V DC from my main battery pack. I dont think you would get much heat from 12V and it would probably also drain the 12V battery right away.

In this way, it is pretty much the same as your diagram, except for the following:

  • where you have 12V going into the relay at connection 30, that is high voltage from battery pack instead. If your battery pack is not around 120V plus or minus, then that might not work.
  • your relay needs to be able to handle the high voltage, which means that a normal automotive relay will not work
  • switching high current DC is hard, which i found out the hard way. My first attempt completely fried the relay the first time I tried to open it (after the initial closing).
  • I talk about this more in one of the comments above
  • I actually put a relay on the high side and the low side of the heater core as a safety feature. this is because i was using the HV traction voltage and that was coming into the cab via the heater core. So when the heater is off, both the high and low side are completely disconnected

Unfortunately I never made a diagram or great notes about this. I think I added the heater switching later, after I had the truck running. I did find a little more info though. The relay I used is KUEP-3D55-12. Here is a link to one, although this is not where I got mine. https://casadelgato.com/kuep-3d55-12

I also paste below a text file which describes the use of the "snubber" or "quencharc" which is basically a cap and a resistor that helps reduce arcing when turning off the relay. I dont recall exactly what I did here. I think i actually purchased a device called a "quencharc", but I couldn’t find any record of it.

This sounds like a lot of trouble, but trust me, if you are switching high voltage DC, you are definitely going to fry the relay if you dont take these steps.

I hope that helps you. The snubber circuit discussion is below. This is a post from EVDL long ago and is no longer on the site. I found it very helpful at the time I was working on my heater. I don't know who the original author is or I would give credit.


        existing fuse
        and switch for                 D1
        heater system               50v 10a
  +12v___/\  ___/ ____________________|\|_____to existing heater
           \/            |            |/|     fan speed switch
        fuse switch      |  S1 overtemp       and blower motor
                          \ cutout switch
                         |  or thermal fuse (on heating element)
                        _|_    |_
                   D2 \_\_/_    _|
             24v zener  /_\ \   _| K1
  gnd____________________|_____|   heater contactor coil (12v)

        pack voltage fuses
        and contactors (at   K1 NO1
        least 1 of each)!    contact
  +120v__/\  ___/______________/ ______________________
  pack     \/           |            |     |     |     |
        F1    S2        |__/\/\__||__|     |     |     |
        fuse  switch or          ||       _|_____|_____|_  five-
              contactor     R1   C1      | |     |     | | terminal
                       22 ohms  0.47uF   | 1  2  3  4  5 | ceramic
                            R2   C2      |____|_____|____| heating
                         __/\/\__||__         |     |      element
                        |        ||  |        |     |
  -pack_________________|______/ ____|________|_____|
                             K1 NO2

The exact values for the parts will depend on your pack voltage and
heater power. I'll give the reason for each part, and typical values for
an EV with a 120vdc pack and a ceramic heating element.

D1  diode, 50v 10a minimum. The fan's PM motor becomes a generator
    as it spins down when you turn the car off. Without D1, the
    generated voltage makes the heater relay K1 drop out slowly,
    which burns and damages the high voltage DC contacts. Hint:
    Use one diode of a bridge rectifier for D1; they're easy to
    find and have an isolated case with quick-connect terminals
    for easy hookup. Example: www.mouser.com #512-GBPC2502 $1.96.

D2  bidirectional zener diode, approximately 24v. Its purpose is
    to let K1's coil voltage kick up to 24v on turn-off. This is
    low enough to protect the heater control switch, but high
    enough to make K1 drop out fast. An ordinary diode across K1's
    coil is the cheap route; but it significantly slows down the
    turn-off of K1, increasing wear on the high voltage DC contacts.
    Example: www.mouser.com #511-1.5KE24CA $0.54.

S1  This is the bimetal overtemperature cutout switch or fuse that
    normally comes already in place on the ceramic heating element.
    Its purpose is to shut off the power if the element overheats
    (for example, if the fan fails or the air passage is plugged).
    The one that comes on the heater is for AC or low-voltage DC
    only! It will *NOT* switch high voltage DC, so it has to be
    wired to cut power to the heater relay coil.

F1  High voltage DC fuse (discussed earlier in this thread).
    Normally, your EV will have a main fuse, circuit breaker,
    contactor etc. that cuts *all* power. Heater fuse F1 is wired
    *after* these main disconnects. This is done so F1 won't have
    to be the one that interrupts 1000+amps if there's a short in
    the heater circuit. This lets F1 be a fuse with a lower max
    interrupting current rating. Example: www.mouser.com #504-ABC-15
    (15a 125vdc) $0.82.

S2  Any switch or contactor used here (to enable high voltage DC
    for the heater) has to be rated to switch DC pack voltage.
    Mechanical switches for these voltages are rare; your best
    bet is an antique home light switch that has DC ratings (from
    the days when some homes had Tom Edison's 120vdc wiring).
    Nowdays it's far more likely to use a contactor for S2.
    In most EVs, the main contactor serves this purpose (the
    heater can't run until the main contactor is turned on).
    But, the controller won't precharge if the heater is on.
    So you may need a separate contactor to enable the heater,
    or a time delay to keep the heater off until the controller
    precharges (not shown).

K1  The high voltage rated relay we've discussed in this thread.
    The Potter & Brumfield PRD or KUEP series are the most common
    choices. They are second-sourced by other companies as well.
    Just be sure you get one with the blowout magnets.

    I show a 2-pole version here, with one pole wired in each
    wire (I prefer to break both sides, rather than put both
    poles in series in one leg, though both work). The common
    ceramic heating elements draw about 9 amps at 120v, so the
    PRD is the better choice. Example: www.mouser.com
    #655-PRD-11DH0-12 $33.57.

C1,C2   Capacitor, 0.47uF 250vac, UL/CSA/CE listed type X2 snubber.
        Don't use a cheap part here; it leads a rough life. It has to
        carry the heater current during the instant that the relay
        contact opens or closes. The exact value is not critical; in
        general, more capacitance is better but costs more. Example:
        www.mouser.com #539-158X474 (Mallory 158 series) $1.86.

R1,R2   Resistor, 22 ohms 1/2 watt carbon composition. It sees high
        peak currents; don't use carbon film. The resistance should
        be roughly equal to the heating element's hot resistance
        i.e. R = 120v/9ohms = 13 ohms) Example: www.mouser.com
        30BJ500-22 $0.23.

You can get R and C combined in one part as a Mallory "Quencharc"
snubber. They cost more and the choice of R and C values is limited, but
it is more convenient to use. Example: www.mouser.com #539-104M06QC22
0.1uF 22ohms $4.42.
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